Transmission system

ABSTRACT

An optical transmission system includes an optical source such as a laser having an optical output, this optical output being modulated such that it has periods of operation having a first set of characteristics interspersed with periods of operation having a second set of characteristics. The output is then split into at least two signals, and one of the signals delayed with respect to the other signal before the two are mixed, such that a portion of the modulated optical output having the first set of characteristics in one of the signals corresponds with a portion of the modulated optical output having the second set of characteristics of another of the signals. This allows a single optical source to provide to a receiver a local oscillator signal simultaneously with a data carrying signal. The system is particularly suitable for lidar applications, including gas sensing, but also has utility in data communications systems.

This invention relates to an optical transmission system. The inventionmore particularly concerns a technique for generating local oscillatorsignals used in coherent communications systems, and other systems thatboth transmit and receive information at optical frequencies.

Conventional coherent optical communications systems, on reception of areceived signal, downconvert the received signal to a frequency at whichit is convenient to process it. This involves mixing the received signalwith a local oscillator signal offset from the received signal by someconvenient frequency. This will produce an output at the difference(downconverted) frequency, where typically lower cost components can beused for the subsequent processing. The advantage of this system overdirect (incoherent) detection is that phase information is maintained inthe downconverted signal, allowing synchronous detection methods to beused, and in addition can give the maximum level of performance inspectral regions where background noise or inefficient detectorsensitivity lead to poor signal recovery.

One system where such optical processing is required is lidar. Here, anoptical signal is transmitted to a target, and the reflections from thetarget are received and then processed to provide information relatingto the target. Generally the transmitted signal is modulated in some waysuch that the returned signal contains additional information about thetarget, such as its range, velocity, vibration or reflectivitycharacteristics etc.

A common method of modulation used in lidar systems is to “chirp” thesignal prior to transmission. A chirp is a frequency sweep of some formapplied to the signal, where the sweep can be continuous or steppeddiscretely, or be a combination of both. A signal that contains a chirpcan be processed in a manner that improves the range resolution of thesystem as compared to a non-chirped signal of equal pulse width.Traditionally, range resolution was improved in lidar systems bynarrowing the transmitted pulse width. If the pulse is narrowed, then tomaintain the same transmitted energy, and hence maintain the same range,the peak power of the pulse needs to be increased, which leads tocostlier and larger systems. A chirped pulse of long duration canemulate the range resolution performance of a short pulse having thesame energy, thus keeping the peak power low without sacrificing rangeperformance. The bandwidth of the chirp is directly proportional to theimprovement in range resolution of the system.

The technique is known as chirp pulse compression (CPC), as the returnedsignal is filtered in a manner that compresses the pulse. Before thefiltering takes place the returned signal is downconverted to a lowerfrequency where signal processing is easier and cheaper. This is done bymixing the returned signal with a local oscillator (LO) that is offsetin frequency from the returned signal by an appropriate amount.

Other modulation schemes include phase modulating the optical signal, orusing some other form of modulation, such as amplitude modulation.

Generation of the local oscillator signal in such transmission systemshas been implemented previously in two distinct ways. In one system asingle laser has been used, the output of which is split into two. Onepart is then put through a modulator to add the modulation before beingtransmitted and reflected from a target, whilst the other is unmodulatedand provides the LO signal. These two signals are then mixed together toproduce the downconverted signal. This is described in more detail inHulme, et al, Optical and Quantum Electronics, Vol 13, p35 (1981) Analternative approach is to use two lasers; one acts as the constantfrequency local oscillator and the second is modulated and thentransmitted as before. One example of this is the Firepond laser radarsystem, detailed in “Laser Radar” by A. Jellalian, Artech House, Boston(1992). The first technique has the disadvantage that the chirpbandwidth is limited to what can be achieved in an external (to thelaser) modulator. External modulators are more limited in theirmodulation capabilities as compared to modulating a beam in the laseritself. The second method requires two separate lasers, and can sufferfrom instabilities due to frequency drift between them.

Optical signals are also used in communications systems, such as intelecommunications, and digital and analogue data transmission systems.These systems generally employ fibre optic transmission media, althoughsome systems do transmit signals over free space. The detection of suchsignals is usually carried out in an incoherent manner, where no LO isrequired, which puts limitations on the minimum channel spacing.

According to the present invention there is provided a transmissionsystem comprising a optical source having an optical output, thisoptical output being modulated such that it has periods of operationhaving a first set of characteristics interspersed with periods ofoperation having a second set of characteristics, wherein the modulatedoptical output is split into at least a first and a second signal, thefirst signal delayed by an amount of time relative to the second signalbefore being mixed with the second signal such that a portion of themodulated optical output having the first set of characteristics in thefirst signal corresponds with a portion of the modulated optical outputhaving the second set of characteristics of the second signal.

Preferably the optical source is a laser.

The present invention allows a single laser to be used to produce boththe LO signal and the modulated signal for transmitting. This providesadvantages over the prior art in that the modulation may be producedwithin the laser, which allows for large bandwidth signals to begenerated without having to provide and stabilise a second laser. Someapplications advantageously employ an additional modulation stage in theform of an external modulator to further modulate the optical output.The external modulator may be positioned before the optical signal issplit into a first and a second signal, or may be positioned such thatit operates exclusively on either the first or the second signal. Theexternal modulator may be an acousto-optic modulator, electro-opticmodulator, or photoelastic modulator. Other modulator types may besuitable.

Preferably the delay is realised using a length of fibre optic cable.The signal to be delayed is launched down a fibre, and will emerge fromthe other end with a delay proportional to the length of the fibre.Preferably the fibre is a single mode fibre. The fibre may be apolarisation preserving fibre.

It should be noted that as used herein the term delay is used in thesense of ensuring that the transit duration of one signal compared withthe other is appropriate to ensure that the first set of characteristicson one signal are mixed with the second set of characteristics on theother. It should not be taken as implying that one signal must have agreater duration of transit. For instance in a lidar application onesignal may be transmitted to and received reflected from a target beforebeing passed to a combiner where it is mixed with the other, reference,signal. The delay line may advantageously be employed on the referencesignal to ensure that the two signals mix appropriately but the actualtransit time of the transmitted signal may be greater or lesseraccording to the particular arrangement.

Advantageously the delay time of the delay line may be changed accordingto system requirements. This may be done by switching extra delays in orout of the total delay line as appropriate, or by any other suitablemeans.

For telecommunications applications the invention allows the system thatgenerates the modulated signal to also supply the local oscillator thatis to be used in the system that demodulates the received signal. Thefirst signal may be delayed with respect to the second, and thencombined with it to form a composite signal. In this way, the inventionprovides the ability to send the part of the first signal having thefirst set of characteristics at the same time as the part of the secondsignal having the second set of characteristics. In this way, a signalis transmitted simultaneously with its LO, allowing for simpledemodulation.

Such a system sends two copies of each of the first and second sets ofcharacteristics, one following the other. Thus any data modulated ontothese signals is actually sent twice, allowing improved error detectingor correcting means to be employed at the receiver.

Alternatively, a signal from the laser comprising a first and a secondset of characteristics may be transmitted to a remote location, wherethe received signal is then split into two, and one of the split signalsdelayed with respect to the other such that the first set ofcharacteristics from the first signal combine with the second set ofcharacteristics from the second signal. This will then providesimultaneously to a demodulator a modulated signal and its LO signal,again allowing for simple demodulation.

For many applications the first set of characteristics of the opticaloutput will be that the optical output is kept at a constant frequencyand constant amplitude. This can then be used as a constant frequency LOsignal as is commonly employed in communications and lidar systems. Someapplications may advantageously employ a first set of characteristicsincorporating a signal that changes its frequency or amplitude, orcombinations thereof, or other characteristics. This may be desirable toremove some known artefact of the signal added by the transmissionsystem.

The second set of characteristics will typically comprise the modulationdesired on the output signal. For example, the signal may be chirped,amplitude modulated, angle modulated, or modulated in any other suitablemanner during this period.

It should be noted that either or both of the first and second sets ofcharacteristics can be changed over time. For example, a datatransmission system that codes the data into the second characteristicperiod will in general transmit different data in each secondcharacteristic period.

The laser may be any type of laser suitable for the application. Whenused in a lidar application the laser is preferably capable of beingcontrolled so as to modify its optical output in terms of amplitude orwavelength according to that required by the system characteristics.Typically the controlling signal will be generated using a computersystem, but other methods, e.g. a hardware waveform generator, may beemployed. Telecommunications systems may advantageously employ one ormore external modulators in which the amplitude or wavelength of theoptical signal from the laser may be changed. The laser may be asemiconductor laser. The semiconductor laser may be modulated bycontrolling the drive current to the laser.

The time delay of the delay line is arranged to be appropriate for thesystem. In a lidar application the delay is preferably arranged to be atleast that of the round-trip flight time of an electromagnetic signallaunched from the lidar system that reflects from a target at the systemmaximum range. Preferably the delay of the delay line is increased toallow for the time taken for the signal to pass through the systemtransmission and reception optics.

A telecommunications application would preferably have the opticaloutput spending equal periods of time between the first and the secondset of characteristics, so providing maximum temporal efficiency, assubstantially all of the signal having the first characteristic can bemixed with substantially all of the signal having the secondcharacteristics. In this circumstance the delay time would be set to thesubstantially the same time period as either the first or second set ofcharacteristics.

The duration of both the first and second set of characteristics isarranged to be appropriate for the system. In a lidar application theduration of each will be governed by, amongst other factors, the lidarsystem's minimum and maximum ranges. The chosen time delay provided bythe delay line to the first signal relative to the second signal mayalso be affected by system parameters including minimum and maximumrange requirements.

A telecommunications application will have a delay preferably equal tothe time duration of a packet of data. In the context of the currentinvention, a packet of data is the data present on a single period ofthe output waveform during either the first or second set ofcharacteristics.

Mixing, of the first and second signals may be performed by any standardmeans. Demodulation and recovery of the modulating signal may beperformed by any standard means, and such methods will be familiar tothose skilled in the art. The first and second signals may be combinedbefore being mixed. Preferably the combination process matches thepolarisation of the first and second signals. The polarisation may bematched using a mechanical polarisation control device, or by anelectro-optic polarisation control device, or by any other suitablemeans.

According to another aspect of the invention there is provided a lidarsystem comprising an optical source having an optical output, thisoptical output being modulated such that it has periods of operationhaving a first set of characteristics interspersed with periods ofoperation having a second set of characteristics, wherein the opticaloutput is split into at least a first and a second signal, the secondsignal being transmitted and received as a returned second signal, andthe first signal delayed by an amount of time relative to the returnedsecond signal before being mixed with the returned second signal suchthat a portion of the optical output having the first set ofcharacteristics in the first signal mixes with a portion of the opticaloutput having the second set of characteristics of the returned secondsignal.

According to another aspect of the invention there is provided atelecommunications system comprising an optical source having an opticaloutput, this optical output being modulated with a modulating signalsuch that it has periods of operation having a first set ofcharacteristics interspersed with periods of operation having a secondset of characteristics, wherein the modulated optical output istransmitted to a remote location where it is received and demodulatedusing a demodulator to reproduce the modulating signal;

-   -   and at a point between generation of the optical output and        demodulation the modulated optical output is split into at least        a first and a second signal, the first signal delayed by an        amount of time relative to the second signal such that a portion        of the optical output having the first set of characteristics in        the first signal corresponds with a portion of the optical        output having the second set of characteristics of the second        signal to aid the reproduction of the modulating signal.

The splitting of the optical output into a first and a second signal,and the delaying of one of the signals relative to the other may takeplace within the transmitter or within the receiver. If it occurs in thetransmitter then preferably the signals are combined before beingtransmitted to a remote location.

According to another aspect of the invention there is provided a gassensor comprising a transmit part and a receive part, wherein thetransmit part comprises an optical source having an optical output, thisoptical output being modulated such that it has periods of operationhaving a first set of characteristics interspersed with periods ofoperation having a second set of characteristics, this optical outputbeing split into at least a first and a second signal, the second signalbeing delayed and combined with the first signal to produce a transmitsignal such that the second set of characteristics is substantiallycoincident in time with the first set of characteristics, and thereceive part comprises a detector capable of distinguishing the firstand second sets of characteristics on receipt of the transmit signal.

Preferably the first and second sets of characteristics are of equaltime duration. Preferably the first set of characteristics comprise aconstant frequency. Preferably the second set of characteristicscomprise a constant frequency, different to that of the first set ofcharacteristics.

According to a further aspect of the invention there is provided amethod of transmitting an optical signal, comprising:

-   -   providing an optical source having an optical output;    -   modulating the optical output with a modulating signal such that        it has periods of operation having a first set of        characteristics interspersed with periods of operation having a        second set of characteristics;    -   passing the modulated optical output to a receive part;    -   demodulating the received modulated optical output in the        receive part to substantially reproduce the modulating signal;        and at a point between generation of the optical output and        demodulation the modulated optical output is split into at least        a first and a second signal, the first signal delayed by an        amount of time relative to the second signal such that a portion        of the optical output having the first set of characteristics in        the first signal corresponds with a portion of the optical        output having the second set of characteristics of the second        signal to aid the reproduction of the modulating signal.

The splitting of the modulated optical output may take place eitherbefore or after the modulated optical output is passed to the receivepart.

Embodiments of the current invention will now be described, by way ofexample only, with reference to the accompanying illustrative drawings,in which:

FIG. 1 diagrammatically illustrates a block diagram of oneimplementation of a lidar system of the prior art.

FIG. 2 diagrammatically illustrates a block diagram of anotherimplementation of a lidar system of the prior art.

FIG. 3 diagrammatically illustrates a block diagram of one embodiment ofthe transmission system according to the current invention.

FIG. 4 diagrammatically illustrates a laser output waveform applicableto the current invention used in the embodiment of FIG. 2

FIG. 5 diagrammatically illustrates a second embodiment of a systemaccording to the current invention, this being a telecommunicationssystem.

FIG. 6 diagrammatically illustrates a third embodiment of a systemaccording to the current invention, this being a gas sensing system.

FIG. 1 shows part of a lidar system of the prior art that incorporates asingle laser 1 whose optical output 5 is split into a first and a secondoptical signal using a semi-silvered mirror 4. The first optical signal6 is then passed through a modulator 2, transmitted via an opticalsystem such as a lens 7 to a target (not shown), and the reflectedsignal 8 from the target is received via an optical system such as alens 9 and then combined with the second optical signal 10 of theoptical output 5 using a second semi-silvered mirror 11. The combinedsignal 12 is then detected using standard detecting means 3. In thisway, the laser supplies both the local oscillator signal 10 and (via theexternal modulator 2) the coded signal for transmit. Typically, thecoded signal comprises a linear ramp in frequency with time. Thedisadvantage of this system topology is that current external modulatorcapabilities provide a limitation on what modulation may be applied. Theusual method of imparting a frequency shift with an external modulatoris to use an acousto-optic modulator. Modulators based upon thistechnology are limited in bandwidth due to the fact that the differentfrequencies are diffracted at different angles. This angular shift meansthat the bandwidth has to be kept low in order to maintain asufficiently small angular spread in the modulated beam. This limits thebandwidth, and consequently the resolution, which can be obtained.

FIG. 2 shows an alternative embodiment of a lidar system of the priorart. Here, two lasers 13, 14 are used to provide the modulated transmitsignal and the local oscillator signal respectively. Again, themodulated signal is typically a frequency ramp signal. The modulatedoutput 15 of the first laser 13 is split into a first and second opticalsignal using a semi-silvered mirror 16, with the first optical signalbeing transmitted to the target via optics 17, and the second opticalsignal going to a reference detector 49. The signal 18 received backfrom the target is transmitted via optics 19 to the signal detector 20,along with the local oscillator signal 21′ from laser 14. A part 21 ″ ofsignal 21 is split off and sent to the reference detector. The output ofthe reference detector is used to control the offset frequency betweenlasers 13 and 14. This embodiment suffers from the problem that,although the laser can be modulated easily, maintaining a knownfrequency difference between the two, so as to get a predictable signalfrom the detector is difficult, and requires extra hardware, as the twoseparate lasers are prone to drift in frequency relative to each other.

An embodiment of the current invention applied to a lidar system isgiven in FIG. 3. A laser 22 provides an optical output 23 to a two waysplitter 24. The optical output 23 is arranged to remain at a constantfrequency for a first time period, after which it is modulated in theform of a frequency ramp for a second time period, repeating thisbehaviour in successive cycles. One output 25 of the splitter 24 is fedinto a delay line 27 which comprises a length of single mode opticalfibre. The other output 26 of the splitter 24 is directed towards anoptical shutter 28, in this case a pulsed acousto-optic modulator (AOM)that is able to selectively gate or window in time, a portion of theoptical signal entering it. Ideally, it acts as an amplitude modulatorhaving a modulation index of 100%. The shutter 28 is controlled bycircuitry not shown on the diagram such that it passes a chirped portion29 of the laser radiation 26 through to the front end 30 of the lidarsystem, where it is transmitted to a target. The signal can be furthermodulated before transmission if desired. For example, data can be addedto the signal, or the signal can be further modulated to provideadditional information content to the received signal.

The returned signal 33 reflected back from the target is passed througha receive optical system 31 to a combiner 32. The second input to thecombiner is the output 38 of the delay line 27. The time delay of thedelay line is arranged to be such that when the signal 38 is combinedwith the signal 33, there exists a time shift between the two signals38, 33, ensuring that the first set of characteristics on the one signalis coincident in time with the second set of characteristics on theother signal. Therefore, the returned chirped signal from the targetgets combined with the constant frequency signal. The combined signal ispassed to the detector 34, where signal detection takes place. Thepresence of a constant, known frequency acting as the LO signal improvesthe signal analysis process later in the processing chain.

The laser 22 used is an external cavity semiconductor laser, andfrequency control is effected by controlling the drive current to thedevice. A linear ramp in the current produces an approximately linearramp in frequency. The drive current waveform can of course be adjustedto provide any desired frequency profile given knowledge of the lasercharacteristics. Hence, any non-linearities can be corrected.

FIG. 4 shows a typical output waveform 37 that can be used with thecurrent invention. FIG. 4 a shows the waveform 37 having a firstcharacteristic 35, this characteristic being a constant frequency, andis the local oscillator (LO) frequency to be used in the demodulationprocess. The waveform has a second characteristic 36 following after aperiod t₁ of the first characteristic 35, the second characteristicbeing a frequency chirp of period t₂. FIG. 4 bshows a waveform 37′, thisbeing the waveform 37 of FIG. 4 a having been delayed by a delay ofsomewhere between t₁ and t₂, this delay being accomplished by passingthe waveform 37 down the delay line 27. It can be seen that the chirp 36of the waveform 37′ occurs entirely within the time period t1 of thewaveform 37. The delay of the delay line can be anything from zero up towhat is achievable with a practical delay line implementation. In alidar application a delay is added to one of the signals relative to theother by the flight time of the signal to the target and back to thereceiving system. The relative time shift given to the signals when theyare combined in the receive system needs to be considered, so the delaytimes of the delay line and the delay caused by signal flight time to atarget must both be accounted for. Note also that when deciding upon theduration of time periods t₁ and t₂ consideration should be given to theround trip flight time of the optical signal between the system minimumrange and the system maximum range.

As there is an approximate linear relationship between the drive currentto the laser and the frequency output within certain bounds, thevertical axes of the graphs of FIGS. 4 a and 4 b could also be regardedas representing drive current. The output frequency is thus chirped byramping the drive current of the laser.

Applying these signals to the system of FIG. 3, the times t1 and t2, andthe delay time td of the delay line are ideally chosen such that thesignal 33 returned from the target is delayed relative to the signal 38at the output of the delay line so as to keep the characteristic 36 ofFIG. 4 b enclosed timewise within the characteristic 35 of FIG. 4 a forall ranges required of the system. Note that other system constraintswill also impose limitations on these variables. Ensuring a suitablephase relationship between the two signals at the point of them beingcombined may also be achieved by incorporating the delay line in theoptical path of the signal to be transmitted, rather than in the path ofthe signal providing the reference. Such a system would result in thedelay line 27 of FIG. 3 appearing in the upper optical path rather thanthe lower optical path as shown in the Figure.

FIG. 5 shows the current invention incorporated into atelecommunications system. An optical signal 41 is produced that has afirst and a second characteristic, where the second characteristiccomprises the optical signal modulated according to a data stream. Themodulation can occur either in the laser 39 producing the opticalsignal, or externally using a separate optical modulator 40. The firstcharacteristic is chosen such that it can act as a convenient LocalOscillator (LO) signal when it is required to demodulate the secondcharacteristic of the signal 41. The signal 41 is then transmitted tosome remote point, where it is first split into two receive signals, 4546 using a splitter 43. One of the signals 45 is passed through a delayline 44, to produce a delayed version 47. The delay time is chosen suchthat the second characteristic of signal 47 is made to coincide in timewith the first characteristic of signal 46. The signals 46 and 47 arethen fed into a demodulator 48, where a copy of the original data streamis extracted.

FIG. 6 shows a further embodiment of the current invention employed as agas sensor. Here, the gas sensor comprises a lidar system 50 which isarranged to transmit a signal 51 through a target gas cloud 52 andreceive any reflected signals. The signal is arranged to comprise atleast two different wavelengths, these being chosen such that one ofthem is known to be coincident with an absorption feature of the targetgas species, and the other to remain relatively unaffected by the gasspecies. The relative attenuation of the two frequencies at the receiverthen gives a measure of the concentration of the target species in theoptical path.

Methods of the prior art either employ two lasers to generate the twodifferent wavelengths, or send a signal at a first wavelength followedby a signal a second wavelength. The latter has the disadvantage thatbetween sending the first and second signals the characteristics of thegas cloud may change.

The transmit section of the gas sensor lidar of the current inventioncomprises a laser 53 that produces a signal 54 alternating in timebetween two different wavelengths. This signal is split between twopaths, one of the paths incorporating a delay line, and then recombined.The delay period is arranged such that when the signals are combined incombiner 55 the signal comprises the two wavelengths simultaneously.This signal is then transmitted to the target gas cloud 52 and reflectedback from a reflector 56. The reflected signal 57 is then passed to areceiver (not shown) where the relative amplitudes of the twowavelengths are compared. FIG. 6 shows the system operating intopographic mode, where a solid target 56 is used as a reflector and theintervening optical path analysed. Distributed backscatter fromparticles making up the gas cloud may also be used to obtain a returnsignal 57.

Other embodiments and applications of the present invention will beapparent to the skilled person.

1. A transmission system comprising a optical source having an opticaloutput, this optical output being modulated such that it has periods ofoperation having a first set of characteristics interspersed withperiods of operation having a second set of characteristics, wherein themodulated optical output is split into at least a first and a secondsignal, the first signal delayed by an amount of time relative to thesecond signal before being mixed with the second signal such that aportion of the modulated optical output having the first set ofcharacteristics in the first signal corresponds with a portion of themodulated optical output having the second set of characteristics of thesecond signal.
 2. A transmission system as claimed in claim 1 where thefirst characteristic of the optical output is a constant frequency.
 3. Atransmission system as claimed in claim 1 wherein the delay mechanismcomprises a length of optical fibre.
 4. A transmission system as claimedin claim 3 wherein the optical fibre is single mode
 5. A transmissionsystem as claimed in claim 1 wherein a portion of the second signal isgated for transmission, and substantially all of the gated portion ofthe second signal is mixed with a portion of the first signal.
 6. Atransmission system as claimed in claim 1 wherein the laser is drivenwith a control signal in order to control the optical output frequency.7. A transmission system as claimed in claim 6 where the laser is asemiconductor laser.
 8. A transmission system as claimed in claim 1wherein the optical output is modulated by a modulation means externalto the laser.
 9. A transmission system as claimed in claim 8 where themodulation means is an acousto-optic modulator.
 10. A transmissionsystem as claimed in claim 8 where the modulation means is anelectro-optic modulator.
 11. A transmission system as claimed in claim 8where the modulation means is a photoelastic modulator.
 12. Atransmission system as claimed in claim 1 wherein the signals arecombined before being mixed.
 13. A transmission system as claimed inclaim 1 where the polarisation of the first signal is matched to that ofthe second signal before being mixed.
 14. A transmission system asclaimed in claim 13 where the delay line incorporates a polarisingpreserving fibre.
 15. A transmission system as claimed in claim 13 wherethe polarisation is matched using a mechanical polarisation controldevice.
 16. A transmission system as claimed in claim 13 where thepolarisation is matched using an electro-optic polarisation controldevice.
 17. A transmission system as claimed in claim 1 where the systemis a lidar system.
 18. A lidar system comprising an optical sourcehaving an optical output, this optical output being modulated such thatit has periods of operation having a first set of characteristicsinterspersed with periods of operation having a second set ofcharacteristics, wherein the optical output is split into at least afirst and a second signal, the second signal being transmitted andreceived as a returned second signal, and the first signal delayed by anamount of time relative to the returned second signal before being mixedwith the returned second signal such that a portion of the opticaloutput having the first set of characteristics in the first signalcorresponds with a portion of the optical output having the second setof characteristics of the returned second signal.
 19. A gas sensorcomprising a transmit part and a receive part, wherein the transmit partcomprises an optical source having an optical output, this opticaloutput being modulated such that it has periods of operation having afirst set of characteristics interspersed with periods of operationhaving a second set of characteristics, this optical output being splitinto at least a first and a second signal, the second signal beingdelayed and combined with the first signal to produce a transmit signalsuch that the second set of characteristics is substantially coincidentin time with the first set of characteristics, and the receive partcomprises a detector capable of distinguishing the first and second setsof characteristics on receipt of the transmit signal.
 20. A gas sensoras claimed in claim 20 wherein each set of characteristics comprise aconstant frequency, where the frequency of the first is different to thefrequency of the second.
 21. A telecommunications system comprising anoptical source having an optical output, this optical output beingmodulated with a modulating signal such that it has periods of operationhaving a first set of characteristics interspersed with periods ofoperation having a second set of characteristics, wherein the modulatedoptical output is transmitted to a remote location where it is receivedand demodulated using a demodulator to reproduce the modulating signal;and at a point between generation of the optical output and demodulationthe modulated optical output is split into at least a first and a secondsignal, the first signal delayed by an amount of time relative to thesecond signal such that a portion of the optical output having the firstset of characteristics in the first signal corresponds with a portion ofthe optical output having the second set of characteristics of thesecond signal to aid the reproduction of the modulating signal.
 22. Atelecommunications system as claimed in claim 21 wherein the modulatedoptical output is split into at least a first and a second signal beforethe signals are transmitted to the remote location.
 23. Atelecommunications system as claimed in claim 21 wherein the modulatedoptical output is split into at least a first and a second signal afterthe signals are transmitted to the remote location.
 24. A method oftransmitting an optical signal, comprising: providing an optical sourcehaving an optical output; modulating the optical output with amodulating signal such that it has periods of operation having a firstset of characteristics interspersed with periods of operation having asecond set of characteristics; passing the modulated optical output to areceive part; demodulating the received modulated optical output in thereceive part to substantially reproduce the modulating signal; and at apoint between generation of the optical output and demodulation themodulated optical output is split into at least a first and a secondsignal, the first signal delayed by an amount of time relative to thesecond signal such that a portion of the optical output having the firstset of characteristics in the first signal corresponds with a portion ofthe optical output having the second set of characteristics of thesecond signal to aid the reproduction of the modulating signal.
 25. Amethod as claimed in claim 24 wherein the splitting takes place beforethe modulated optical output is passed to the receive part.
 26. A methodas claimed in claim 24 wherein the splitting takes place after themodulated optical output is passed to the receive part.